The present invention relates to an electrostatic display device having a fixed electrode, a resilient sheet electrode, and an insulating layer interposed between the fixed and the sheet electrodes, whereby upon the application of a voltage between the two electrodes, the resilient sheet electrode is attracted to and covers the outer surface of the fixed electrode due to an electrostatic attractive force generated between the electrodes, thus changing the appearance of the device.
The principle employed in such an electrostatic display device is as follows:
The application of a voltage V between the fixed electrode and the sheet electrode with a dielectric layer therebetween produces an electric field between the electrodes, which causes polarization in the dielectric layer. The polarized dielectric layer in turn generates an electrostatic attractive force between the dielectric layer and the fixed electrode and between the dielectric layer and the sheet electrode. Thus, when the electrostatic attractive force is large enough to attract the sheet electrode toward the fixed electrode against the resiliency of the sheet electrode, the sheet electrode covers the outer surface of the fixed electrode, thereby providing a distinct change in the appearance of the device.
The attractive force for attracting the sheet electrode per its unit area is represented as follows: EQU F=CKV/d
where, K is the dielectric constant of the dielectric layer;
V is the voltage applied between the electrodes; PA1 d is the thickness of the dielectric layer; and PA1 C is a constant related to the device.
U.S. Pat. No. 3,897,997 to Kalt discloses an electrostatic display device of this type for practical use. Such device, referring to FIG. 8, includes a fixed metal electrode 51 which has a cylindrical outer surface and is fixed to an insulative base 52, and a resilient sheet electrode 54 which is fixed to the base 52 at its one end 53. The sheet electrode 54 is mounted adjacent to the fixed electrode 51 in contact therewith at a portion P. The outer surface of the fixed electrode 51 or the inner surface of the sheet electrode 54, or both, are covered or coated with a thin layer of an insulating material 55 as a dielectric layer.
The sheet electrode 54 consists of, for example, a resilient polymer film 56 as a core of the electrode, such as a polyethylene terephthalate film with an electrically conductive metal like aluminum 57 vapor-deposited thereon.
A d.c. power supply 58 is connected to the fixed electrode 51 and the sheet electrode 54 through lead wires 59 and 60, respectively, so that a d.c. voltage is applicable between the electrodes.
When there is no voltage between the electrodes 51 and 54, the sheet electrode 54 extends straight upwardly because of its resiliency. Thus, the outer surface of the fixed electrode 51 can be seen from above. However, the application of a voltage between the electrodes causes the sheet electrode 54 to be pulled toward the fixed electrode 51 and covers the outer surface thereof in a moment as shown in the double dot chain lines. Now the outer surface of the sheet electrode 54 can be seen from above.
Since the sheet electrode 54 flaps in this way upon applying a voltage between the electrodes 51 and 54, various types of display can be realized when the appearance of the outer surfaces of the two electrodes are different from each other, for example, in their reflectivity, color, patterns and messages they carry.
The above mentioned electrostatic display device is expected to be used in a wide variety of displays since it has many advantages. For example, the device has a memory function as well as a complicated display function. Furthermore, the device requires less power. However, it is also true that the device has some disadvantages.
For example, the above device has a fixed electrode of an opaque material, typically of metal, with a painted layer thereon as an insulating or dielectric layer, so that the flapping of the sheet electrode provides a distinct change in appearance of the device. Therefore, the prior device requires that the painted layer per se has an excellent electrical property.
As is apparent from the previously mentioned formula, when K/d is larger, the sensitivity of the device is better. This means that a thinner layer of paint having a larger dielectric constant should be applied to the fixed electrode. From a practical point of view, the fixed electrode should have such a thin layer of paint that it does not remarkably reduce the attractive force between the two electrodes when a voltage is applied therebetween. Usually, a painted layer of from several to several tens of microns in thickness is applied, and the layer is required to be sufficiently insulative as well as sufficiently durable. Otherwise there will be a possibility, if small, that the sheet electrode in part welds to the fixed electrode. This prevents the sheet electrode from returning to its original position upon the removal of a voltage between the electrodes.
However, a paint or coloring agent which meets such severe requirements is available only with difficulty, and would be very expensive. It will be helpful to apply a thicker layer of paint on the fixed electrode for increasing the insulative property of the layer. But, a thicker layer of paint requires a larger voltage to attract the sheet electrode to the fixed electrode. This reduces the usefulness of the device with respect to power consumption. The application of a higher voltage brings about other disadvantages, and thus is far from a practical application of the device.